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Drilling Fluid:
Hole cleaning Hole cleaning is one of the basic functions of any drilling fluid. Cuttings generated by the bit, plus any caving and/or sloughing, must be carried to the surface by the mud. Failure to achieve effective hole cleaning can lead to serious problems, including stuck pipe, excessive torque and drag, annular pack-off, lost circulation, high mud costs and slow drilling rates. Cuttings transport is affected by several interrelated mud and drilling parameters. Removing cuttings from below the drill bit is still a crucial function of a drilling fluid. The circulatory fluid rising from the bottom of the well bore carries the cuttings toward the surface. Under the influence of gravity, these cuttings tend to fall through the ascending fluid. This is known as slip velocity. The slip velocity will depend upon the viscosity (thickness) and density of the fluid. The thicker the fluid, the lower the slip velocities. The more dense the fluid, the lower the slip velocity. For effective cuttings removal, the fluid velocity must be high enough to overcome the slip velocity of the cuttings. This means that fluid velocity can be lowered in a highly viscous (thick) or very dense fluid and cuttings still effectively removed from the well bore. The density of a fluid is determined by other factors and is not usually considered a factor in hole cleaning; therefore we limit adjustment of hole cleaning properties to viscosity and velocity adjustments to the drilling fluid. The viscosity desired will depend upon the desired hydraulics and the size of the cuttings contained in the fluid. The velocity will depend on several factors -the pump (capacity, speed, efficiency), the drill pipe size and the size of the bore hole. The velocity of a fluid will determine its flow characteristics, or flow profile.
There are five
stages, or different profiles, for a drilling fluid: The ideal velocity is one that will achieve laminar (or streamline) flow because it provides the maximum cuttings removal without eroding the well bore. On the other hand, turbulent flow (resulting from too high a velocity or too low fluid viscosity) not only requires more horsepower but can cause excessive hole erosion and undesirable hole enlargement. The proper combination of velocity and viscosity is a must for the right hydraulics and efficient hole cleaning. Cuttings will have a tendency to collect at points of low fluid velocity in the well bore annulus. These areas are found in washouts and where the drill pipe rests against the wall of the well bore. To that end, it is a good practice to rotate and work (raise and lower ) the drill string while just circulating to clean the hole, as this will help keep the cuttings in the main flow of the fluid and not allow them to gather next to the wall or pipe. Hole angle, annular velocity and mud viscosity are considered to be the most important. Cuttings and particles that must be circulated from the well have three forces working on them:
(1) a downward force
due to gravity,
Free settling
occurs when a single particle falls through a fluid without interference
from other particles or container walls. The larger the difference between
the density of the cutting and the density of the liquid, the faster the
particle will settle. Hindered settling occurs when fluid displaced by falling particles creates upward forces on adjacent particles, thereby slowing down their settling rate. The net results is still an overall downward movement, but the settling rate is always less (hindered), thus the name. Boycott settling is an accelerated settling pattern that can occur in inclined well bores. Boycott settling is the consequence of rapid settling adjacent to the high and low sides of inclined well bores. This causes a pressure imbalance which drives the lighter, upper fluid upwards and any cutting beds on the low side downwards. At relatively low flow rates, mud flows mainly along the high side and accelerates or enhances the Boycott effect. High flow rates and pipe rotation can disrupt the pattern and improve hole cleaning. If not properly supported, cuttings can accumulate at the bottom of the hole or on the low side of inclined intervals. "Plugs" and stuck pipe can be caused by dragging bottom hole tools up through pre-existing beds. Cuttings accumulations can be difficult to erode or re-suspend, so mud properties and drilling practices which minimize their formation should be emphasized. Cuttings transport efficiency is largely a function of annular velocity and the annular velocity profile. Increasing annular velocity will always improve hole cleaning, though it still must work with other hole parameters. In fully concentric annulus, flow is evenly distributed around the drill string. Thus there is an equal distribution of fluid energy for cuttings transport. However, the drill string tends to lay on the low side of the hole in inclined sections, shifting or skewing the velocity profile, the results of which is not conducive to cuttings transport. Cuttings accumulate on the bottom of the hole adjacent to the drill pipe where the mud flow is minimal. In this situation, pipe rotation is critical to achieve effective hole cleaning. However, there are times when drilling a directional hole that pipe rotation will not be possible. All is not lost at this point since we can offset the detrimental effects of not rotating with different mud types and changing certain mud properties. Generally speaking, different drilling fluid types provide similar cuttings transport if their down hole properties are similar. Properties of particular interest to hole cleaning include mud weight, viscosity and gel strengths. Mud weight helps buoy cuttings and slow their settling rate but it is really not used to improve hole cleaning. Instead, mud weights should be adjusted based only on pore pressure, fracture gradient and well-bore stability requirements. Mud viscosities helps determine carrying capacity. Yield points historically has been used as the key parameter which was though to affect hole cleaning. More recently, evidence concludes that Fann 6 and 3 RPM values are better indicators of carrying capacity. These values are more representative of the Low Shear Rate Viscosity (LSRV) which affects hole cleaning in marginal situations. One common rule of thumb is to maintain the 3 RPM value so that it is greater than the hole size (expressed in inches) in high angle wells. Gel strengths provide suspension under both static and low shear rate conditions. The ideal situation is for the fluid to have high, fragile gels that develop quickly and are easily broken. Excessive high, progressive gels, on the other hand, should be avoided as they cause high transient pressures that cause a number of serious drilling problems. Listed below are practical hole-cleaning guidelines aimed at field use on directional bores. Use hole-cleaning techniques to minimize cuttings-bed formation and subsequent slumping which can occur in 30-60 degree hole sections. Utilize elevated-viscosity fluids from the start because cuttings beds are easy to deposit but difficult to remove. Maintain LSRV between 1.0 and 1.2 times the hole diameter when in laminar flow. This requirement will be easier to accomplish if the fluid is treated with a super's or high vis. This product is a bio-polymer that elevates the LSRV in fluids. Treat mud to obtain elevated, flat gels for suspension during static and low flow rate periods. Consider using the mud system that will give you excellent LSRV values and superior suspension abilities. The system uses an untreated bentonite and a mixed metal hydroxide additive. Schedule periodic wiper trips and pipe rotation intervals for situations where sliding operations are extensive. Rotate pipe at speeds above about 50 RPM if possible to prevent bed formations and to help remove pre-existing beds. Expect little help from viscous sweeps, unless they are accompanied by high flow rates and pipe rotation |
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